Directions

Bring a pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 3/4 cup of the cooking water, then drain the pasta.

Add the butter to the empty pot and melt over medium-high heat. Add the shallot and cook, stirring, until slightly softened, about 2 minutes. Add the flour and mustard powder and cook, stirring, until the flour begins to toast, about 1 minute. Whisk in the milk and the reserved cooking water; cook, whisking, until thick, about 2 minutes.